In vitro Human Embryo Morphology - An Odissey Outside the Oviduct

Review Article

Austin J Anat. 2021; 8(1): 1099.

In vitro Human Embryo Morphology - An Odissey Outside the Oviduct

Hurtado R1,2, de Lima Bossi R2, Valle M2, Sampaio M2 and Geber S1,2*

¹Department of Obstetrics and Gynaecology of the Medical School, Universidade Federal de Minas Gerais, Brazil

²ORIGEN, Center for Reproductive Medicine, Brazil

*Corresponding author: Selmo Geber, Department of Obstetrics and Gynaecology of the Medical School, Universidade Federal de Minas Gerais, Av Contorno 7747 Belo Horizonte, Minas Gerais, Brazil

Received: May 01, 2021; Accepted: June 09, 2021; Published: June 16, 2021


The authors describe the human embryo development in vitro, during the preimplantation stages, i.e., from the zygote until the blastocyst stage. They also describe the methods to select the most suitable embryo for transfer in cycles of In vitro fertilization treatment, performed by infertile patients, in order to increase implantation and pregnancy rates.


Couples with the inability to conceive after one year of unprotected intercourse turn to infertility treatment and, in many cases, Assisted Reproductive Technologies (ART)/ In vitro Fertilization (IVF), a technique responsible for the birth of more than 8 million babies worldwide since 1978. Embryo early development and morphology hold important information gathered from basic science that can be extensively applied to ART and, therefore, optimize the outcomes.

The lack of treatment options for improving the quality of sperm and eggs is addressed by increasing gamete quantity. For women, multiple follicular developmet, to increase oocyte number, is induced by gonadotropins in a single menstrual cycle. Moreover, in order to increase pregnancy rates, more than one embryo is transferred for the uterus. Therefore, IVF might increase the rates of multiple pregnancies and premature delivery with consequences on public health [1]. There is no debate that a better embryo selection could downsize the number of transfered embryos per cycle, and therefore decrease the incidence of multiple pregnancies [2].

Even after decades of upcoming technologies for better evaluation of embryos, traditional morphology assessment through a binocular microscope is still the first-line method for embryo selection to transfer in IVF cycles, for none of the more recent technologies have been proven superior [3-5]. On the other hand, morphological assessment does not detect chromosomal abnormalities or defects in critical cellular processes and metabolism that could impact the viability of an embryo. For these purposes, the methods being used to evaluate chromosomal abnormalities of embryos are Pre- Implantation Genetic Testing (PGT) [6] after embryo biopsy or using a non-invasive technology; and for embryonic metabolism, technologies collectively called ‘omics’ which may include genomic, proteomic, transcriptomic and metabolomic profiling of the embryos [7]. Chromosomal analysis has been heavilly criticized since its early years using Fuorescence In-Situ Hybridization (FISH) all the way down to modern PGT-A because it is very well documented that embryos are compartmentalized. This means that the DNA of an abnormal cell within an embryo could be present in one part of the embryo and absent in the others [8]. Also, there is a possibility of an auto correction during development [9].

Prospective randomized trials, which would be the most reliable types of studies for generating evidence-based guidelines, are somewhat difficult to perform considering human embryos. This is the case for reasons highlighted by Matchinger in 2013: a) there is no way to establish a direct correlation between specific embryos transfered and viable ongoing pregnancies observed as result; b) Single-embryo transfers are still not widely used across fertility clinics; c) embryo grading criteria and patient selection may vary significantly from clinic to clinic and from study to study. The result for this situation is that most data concerning embryo assessment derives from retrospective and/or small-sampled studies. In recent years, several international scientific forums have been dedicated to standardize embryo quality assessments in order to promote better embryo selection, producing comparable results for different types of studies and, therefore more reliable guidelines.

Morphological Parameters in Embryo Assessment

Currently, embryo selection is based on embryo morphology and the rate of embryo development in culture. This method is subjective because evaluations are based on the number of blastomeres in the embryo, symmetry of the blastomeres and degree of fragmentation. Also, human embryonic development follows a specific sequence of events where morphological characteristics are defined at determined points in time. In result, embryo evaluation can be carried out either by sequential assessments or a ‘cumulative’ one-time approach [10].

More recently a new emerging thechnology is being tested in order to evaluate embryo developmente and improve selection and implantation. Although the use of time-lapse microscopic photography associated to Artificial Intelingence is increasing and gaining more attention there is no data suggesting an outvome improvement for ART.

Zona Pellucida

Zona pellucia is a non-cellular glycoprotein layer surrounding the plasma membrane of oocytes and embryos, secreted during oogenesis. It has been speculated that both the thickness and the diameter of the Zona Pellucida in hatching embryos could be used as markers for embryo quality. It is known that it is easier for invivo- produced embryos to hatch in comparison to in-vitro-produced embryos. These embryos present their zona pellucida with smaller diameter and thickness. It is thought that this is due to a different molecular composition of the zona as a consequence to the exposure to the oviducts fluids, resulting in a more rigid structure that is more prone to crack [11].

Perivitelline space

Perivitelline space is located between the zona pellucida and the plasma membrane. Aside from some very scarce information concerning size and contents in in-vivo and in-vitro produced embryos, researchers have been paying very little attetion to the periviteline space [12] As pointed out by [13] , this space provides information related to culture conditions and therefore can suggest embryo quality. A reduction in periviteline space could translate as swelling of blastomeres, a marker for poor embryo quality directly related to less cell compaction at the morula stage [14].


The first assessment of an embryo should be at the zygote stage for the appearance of two pronuclei (the first sign of successful fertilization). The Pronuclear scoring systems usually assess the number and relative position of the Nucleolar Precursor Bodies (NPB), pronuclear size and aligment (similar size with central location) and appearance of the cytoplasm [15,16] (Figure 1). These features proved to have a predictive relation to blastocyst formation with increased implantation potential [17,18]. These studies were later challenged by robust data showing lack of consistency for pronuclear scoring systems as valid embryo selection criteria [19] (Table 1).